Use Doctrine 2.3 CountOutputWalker

Use Doctrine\ORM\Tools\Pagination\CountOutputWalker if it is available (Doctrine >= 2.3.0) instead of Doctrine\ORM\Tools\Pagination\CountWalker to provide support for counting queries which contain a HAVING clause.
Merge pull request #41 from boite/master

Fix "Cannot count query that uses a HAVING clause" (Doctrine >= 2.3.0)